1. Field of the Invention
The present invention relates to a portable light detecting and ranging (LIDAR) system for long-range aerosol detection of biological weapon gas clouds. As such, the system can be used to provide early warning for field personnel, providing the necessary time for personnel to prepare for the arriving gas cloud.
2. Discussion of the Background
Remote stand-off detection of chemical/biological (chem/bio) agents is considered to be a critical necessity in early warning systems enabling maximum survivability of personnel in the battlefield and other sensitive areas. Pulsed elastic backscatter lidar operating in the visible, as described by Lee, et al, xe2x80x9cMicro Pulse Lidar for Aerosol and Cloud Measurementxe2x80x9d, Advances in Atmospheric Remote Sensing with Lidar, pp 7-10, A. Ansmann, Ed., Springer Verlag, Berlin, 1997, the entire contents of which are incorporated herein by reference, and near IR, as described by Condatore, et al, xe2x80x9cU.S. Army Soldier and Biological Chemical Command Counter Proliferation Long Rangexe2x80x94Biological Standoff Detection System (CP LR BSDS)xe2x80x9d, Proceedings of SPIE, Vol. 3707, 1999, the entire contents of which are incorporated herein by reference, have demonstrated the high sensitivity and long-range (up to 50 km) capability to detect aerosol clouds. Consequently, aerosol lidar is a chosen technique for long-range detection of bio-warfare aerosols. However, single wavelength aerosol lidars, as currently employed, do not provide discrimination between biological weapon (BW) agent aerosols and other natural or interferent aerosol clouds. The capability to differentiate can be augmented by using multiple wavelength and/or multiple polarization elastic scattering signatures. However, the elastic scattering technique lacks the required specificity for deterministic application of the data in the battle field.
Aerosol lidar is an ideal complement to uv fluorescence lidar, as demonstrated by Wilson, et al, xe2x80x9cDevelopment of IR and UV Lidar systems for standoff detection of airborne biological materialsxe2x80x9d Final Report, Contract DAAA15-91-C-0138, STC Technical Report, 1993, the entire contents of which are incorporated herein by reference, which discloses a UV laser that excites fluorescence from the biological constituents of the aerosol and measures the fluorescence signature of the biological constituents to provide specificity for discrimination between bio-aerosols and other naturally occurring or interfering aerosols. Since atmospheric absorption at UV wavelengths is high and fluorescence cross-section of the target particles is small, even the use of a high energy laser source with a large aperture telescope only enables conventional fluorescence lidar to achieve a range coverage of three to four kilometers. Jezek and Cannaliato, xe2x80x9cBiological Standoff Detectionxe2x80x9d, Joint Workshop on Standoff Detection for Chemical and Biological defense, pp. 26-30, October 1998, the entire contents of which are incorporated herein by reference, have been actively developing both long range and short range sensor systems. Long-range biological standoff detection system LR BSDS, as described in Condatore, et al, xe2x80x9cU.S. Army Soldier and Biological Chemical Command Counter Proliferation Long Rangexe2x80x94Biological Standoff Detection System (CP LR BSDS)xe2x80x9d, Proceedings of SPIE, Vol. 3707, 1999, the entire contents of which are incorporated herein by reference, is based on elastic scatter aerosol lidar. Short range biological standoff detection system SR BSDS, as described in Suliga, et al, xe2x80x9cShort Range Biological Standoff Detection System (SR-BSDS)xe2x80x9d, Fourth Joint Workshop on Standoff Detection for Chemical and Biological Defense, Sep. 15, 1998, the entire contents of which are incorporated herein by reference, is based on fluorescence and aerosol lidar.
Current chem/bio defense detection systems can provide a rapid indication of a possible BW attack by utilizing multiple independent technologies to provide separate lines of data, which are less likely to be wrong at the same time, thus reducing false alarms. However, present technologies, owing to the complexity and laser power levels required for fluorescence and aerosol lidar, are limited in range and not well suited for an in-the-field, portable early warning detection system.
Accordingly, one object of the present invention is to provide an early warning detection system which can provide accurate detection of biological weapons at sufficient distance to provide an adequate response time.
Another object of the present invention is to integrate multiple analysis techniques into a detection system to reduce a probability of false alarms.
Still a further object is to provide a self-aligned detection system which improves reliability of the detection system in the field.
A further object of the present invention is to provide a digital detection system which can by multiplexing improve the signal-to-noise ratios and the detected signals.
These and other objects are achieved in a lidar system including a laser which provides laser pulses of at least two wavelengths, a transmitter which transmits the laser pulses, a receiver which receives both elastically backscattered signals from airborne agents and fluorescence signals from the airborne agents, a common telescope which both focuses a laser beam transmission of the laser pulse from the transmitter to a far field and receives the elastically backscattered signals and the fluorescence signals from the far field, a digital detection system having at least one of a backscatter optical detector which detects the elastically backscattered signals and a fluorescence optical detector which detects the fluorescence signals from the airborne agents.
Indeed, the lidar system of the present invention maximizes the signal-to-noise ratio (SNR), thus maximizing the range capability for a given SNR. An acceptable criterion for the confident detection of BW agent aerosol in the atmosphere is for the signal-to-noise ratio of the lidar signal to be about four. In addition to photon shot noise generated by the laser scattered light falling on the detector, a detection system itself can contribute to the noise. In a conventional lidar system, where analog detection technique is used, the noise depends on the detector dark noise together with the signal shot noise, i.e., the noise in the associated amplifier and the detection bandwidth. The bandwidth of a lidar system is determined by the desired spatial resolution of the lidar measurement. For example, a 15 m spatial resolution requires at least 5 MHz bandwidth. The minimum signal required for an analog lidar system to successfully measure an aerosol is determined by the detector dark current and the bandwidth. Thus, in the analog lidar design approach of the present invention, increasing the measurement range requires increasing the signal, which normally implies a high-energy laser and a large telescope for collecting the signal. For the BSDS system previously discussed, a laser energy greater than 100 mJ, and a telescope receiver size of 65 cm dia. is utilized.
In contrast, the portable digital lidar (PDL) system of the present invention is based on a different approach, i.e., digital detection, where the detection noise is minimized so that a much lower signal level is adequate to yield the required SNR. Digital detection utilizes photon-counting which generates digital pulses for every photon that is detected and is not affected by the bandwidth or the amplifier noise. In one embodiment of the present invention, a Geiger mode avalanche photodiode (APD) detector is utilized with low signal induced noise. Other than the photon shot noise, the only noise source in digital detector is the detector dark count noise, which is about three orders of magnitude smaller than the dark current noise in an analog detector.
Hence, digital detection system is capable of detecting signals nearly a thousand times smaller than analog detection. Thus, the laser energy for the fluorescence excitation can be reduced to xcx9c1 mJ, allowing compact diode-pumped solid state (DPSS) lasers to be utilized which have the performance stability and the rugged configuration necessary for battlefield operation along with low power consumption. As a consequence, the laser size and cost are reduced by nearly a factor of ten. Further, the DPSS lasers can operate at high pulse repetition frequency (PRF) of a few kHz without significantly increasing cost or size. Averaging multiple shot data improves the SNR (as the square root of the number of pulses). By averaging over many thousands of shots, the useful range of the lidar is extended as the SNR at the extended range becomes acceptable within a few seconds. While the SNR of an analog lidar detector can be improved by signal averaging, increasing the PRF of a analog-suitable laser (e.g., a 100 mJ laser with a repetition rate of 30 Hz) can be relatively expensive and difficult.
The PDL system of the present invention is equipped with a scanner to cover a wide angle (xc2x160xc2x0) for simultaneously monitoring multi-wavelength elastic scattering and laser-induced fluorescence from aerosols. Tracking of cloud and aerosol packets by rapidly scanning over a wide field of view allows the wind direction and speed to be obtained continuously. The concept of using a single 3rd harmonic Nd:YAG laser and tapping the residual 1.06 xcexcm, 532 nm wavelength outputs for aerosol elastic scatter not only results in a compact lidar system but provides other additional benefits, as discussed next.
First, wavelengths greater than 1.5 xcexcm have been used in other lidars (e.g., LR and SR BSDS) to render the lidar systems eye-safe. However, the 1.5 xcexcm laser is a complex system requiring the 1.06 xcexcm Nd:YAG output to be down shifted in an optical parametric oscillator OPO. Also, commonly available 1.5 xcexcm detectors are not sufficiently sensitive; hence special detectors are needed, adding to the cost and complexity of the sensing systems. On the other hand, since the laser energy required for the PDL of the present invention is small, the laser beams at both 532 nm and 1.06 xcexcm can be made eye-safe by expanding the transmitted laser beam. The expansion of the transmitted laser beam is achieved by utilizing a telescope as both a transmitter and a receiver. Internal analysis of the aerosol backscatter signals have shown that a 20 km range is achieved with such an eye-safe lidar with a minimal averaging time of less than 0.5 sec, so that rapid scanning is feasible.
Second, the particle sizes for naturally occurring aerosols range from 0.2-0.8 xcexcm while the particle size for bio-aerosols range from 2-10 xcexcm. Of the two chosen laser wavelengths, 532 nm is roughly equal to, and 1.06 xcexcm is larger than, the natural aerosols, whereas the two chosen wavelengths are both smaller than the bio-aerosols. Hence, a differentiation between the naturally occurring aerosols and the bio-aerosols is possible by comparing the scattered signals at these two wavelengths. According to the present invention, combining the scattering differences with a 355 nm excited bio-fluorescence which has the potential for discrimination between man-made and naturally occurring bacteria provides an early and confirmatory warning system for the detection of a bio-aerosol presence.
Third, another innovation of the PDL system of the present invention is a common transmitter/receiver telescope system. Since the transmitter and receiver utilize a common optical conjugate point, both the transmitted beam and the receiver field-of-view stay aligned at all times, making the system immune to misalignment resulting from the displacement of telescope optics resulting from either vibration or thermal distortions. A single telescope which serves as both a transmitter and a receiver results in a compact, low-cost, and light system. Further, according to the present invention, internal scattering interferences can be minimized through the use of spatial and spectral filters.
Thus, the PDL lidar sensor of the present invention is well-suited for biological standoff detection. The sensor addresses the requirements of high sensitivity and autonomous operation capability, eye-safety, and the ability to operate during the day for long-range detection. Importantly, these attributes are complemented with small size, rugged packaging, low power and maintenance requirements, and low cost.